Conventionally, microfabrication of semiconductor substrates has been carried out by lithography using a photoresist composition in the production of semiconductor devices. For the microfabrication of semiconductor substrates, there is a known fabrication method in which a resist film formed from a photoresist composition is formed on a substrate to be fabricated such as a silicon wafer, active rays such as ultraviolet rays are applied onto the resist film through a mask pattern with a pattern that is transferred to a semiconductor device, the resist thin film is developed for patterning, and the substrate to be fabricated such as a silicon wafer is etched using the obtained photoresist pattern as a protective film to transfer the pattern to the substrate.
In recent years, highly integrated semiconductor devices require a finer resist pattern. To address this, the active rays used for the patterning of resist films have been likely to have a shorter wavelength from a KrF excimer laser (248 nm) to an ArF excimer laser (193 nm). However, the use of active rays having a short wavelength has a higher possibility of causing significant problems such as resolution reduction due to the effects of irregular reflections of the applied active rays from a substrate and standing waves. In order to solve such a problem, a bottom anti-reflective coating (BARC) has been widely employed between the photoresist and the substrate to be fabricated as a resist underlayer film.
In future, when a finer resist pattern will be further developed, a thinner resist will be required in order to prevent problems such as the resolution reduction and collapse of a resist pattern after development. However, the thinner resist cannot provide a resist pattern film thickness sufficient for the fabrication of a substrate. To address this, there is required a process for imparting a function as a mask during the substrate fabrication to a resist underlayer film that is formed between the resist and a semiconductor substrate to be fabricated in addition to the resist pattern.
As the resist underlayer film for such a process, unlike a conventional resist underlayer film having a high etching rate, a resist underlayer film having a similar selection ratio of dry etching rate to that of a resist, a resist underlayer film having a smaller selection ratio of dry etching rate than that of a resist, or a resist underlayer film having a smaller selection ratio of dry etching rate than that of a semiconductor substrate is used.
There are known resist underlayer film forming compositions including various polymers for forming the resist underlayer film. For example, there are disclosed a resist underlayer film forming composition including polyvinylcarbazole (see Patent Document 1, Patent Document 2, and Patent Document 3), a resist underlayer film forming composition including a fluorene phenol novolak resin (for example, see Patent Document 4), a resist underlayer film forming composition including a fluorene naphthol novolak resin (for example, see Patent Document 5), and a resist underlayer film forming composition including a resin containing a fluorene phenol and an aryl alkylene as repeating units (for example, see Patent Document 6 and Patent Document 7).
There is also disclosed a novolak resin that is used as an insulating material for printed wiring, boards and the like, has high heat resistance, and uses carbazole and acetaldehyde (see Patent Document 8).